WASHINGTON — Here at the annual meeting of the Association of American Geographers, there’s a lot of map talk. People are proudly showing off maps of land rights, maps of cloud formation, even maps of how teenagers form herds in malls.

But to me, one of the most compelling maps traced the steps of a single man, a drug addict, as he traveled through Baltimore.

The man carried a GPS unit, given to him by clinicians who were treating him at a methadone clinic. The unit, about the size of a ...